A difficult obstacle associated with the exploration and production of oil and gas is management of significant ocean currents. These currents can produce vortex-induced vibration (VIV) and/or large deflections of tubulars associated with drilling and production. VIV can cause substantial fatigue damage to the tubular or cause suspension of drilling due to increased deflections.
Two solutions for VIV suppression are helical strakes and fairings. Typically, helical strakes are made by installing fins helically around a cylindrical shell. The cylindrical shell may be separated into two halves and positioned around the tubular to helically arrange the fins around the underlying tubular. While helical strakes, if properly designed, can reduce the VIV fatigue damage rate of a tubular in an ocean current, they typically produce an increase in the drag on the tubular and hence an increase in deflection. Thus, helical strakes can be effective for solving the vibration problem at the expense of worsening the drag and deflection problem.
Another solution is to use fairings as the VIV suppression device. Typical fairings have a substantially triangular shape and work by streamlining the current flow past the tubular. A properly designed fairing can reduce both the VIV and the drag. Fairings are usually made to be free to weathervane around the tubular with changes in the ocean current.
A challenge associated with both helical strakes and fairings is their use on tubulars that have an outside diameter that shrinks due to hydrostatic pressure. This is often true of risers that have insulation or buoyancy on the outside of an inner metallic tubular. Since it is usually much cheaper to install helical strakes or fairings on a tubular while it is above the water surface (before it is lowered), this means that the tubular diameter will often be larger at the surface than at depth. Helical strakes that are banded onto the tubular risk coming loose when the diameter shrinks since the bands are typically not sufficiently compliant to accommodate the diameter change. Fairings utilize thrust collars that restrain the fairings from sliding down the tubular. These thrust collars are often banded on and suffer from the same lack of compliance that helical strakes experience.